A fuel return valve (FRV) is intended to return a certain quantity of hot, excess fuel which has passed through a hot section of a system constituted by an aeroplane engine and its fuel system to a fuel tank of an aeroplane in order to facilitate thermal management of the system.
If carried out at too high a temperature, returning hot fuel to the tank can cause problems with boiling or evaporation of the fuel. Despite such problems, FRVs are valued for their efficiency and low bulk.
In order to overcome the problems above, FRVs have been designed to meter out or deliver a fuel mixture from a hot fuel bleed obtained from the high pressure portion of the system and from cold fuel bled from upstream of the heat sources in the system in a mixing ratio or ratio of the flow rates of the cold fuel and the hot fuel composing the mixture leaving the FRV and returning to the tank of the aeroplane.
Ideally, the mixing ratio of a FRV is constant in order to control the temperature of that mixture irrespective of the pressure of the hot fuel and of the cold fuel at the inlet of a FRV.
Thus, in the prior art, FRVs are known which deliver a constant flow at their outlet and return it to the tank by means of two fixed restrictions. A first fixed restriction is in series with the hot bleed or hot restriction and a second fixed restriction is in series with the cold bleed or cold restriction, the first restriction and the second restriction being connected in parallel over a mixing point connected via a circuit for returning the reduced temperature fuel mixture to the tank.
The pressure in the tank being essentially constant, the pressure at the mixing point is also constant apart from the pressure drop between the mixing point and the tank. This means that the hot restriction of a FRV operates between the pressure of the hot bleed and the constant pressure of the tank, the difference between these two pressures being termed the hot pressure drop. This also means that the cold restriction of a FRV operates between the pressure of the cold bleed and the constant pressure of the tank, the difference between these two pressures being termed the cold pressure drop.
A constant pressure drop in any restriction, being connected to a constant flow rate in said any restriction, means that for a FRV, when the pressure of the hot bleed or hot pressure is constant, the flow delivered by the hot restriction is constant, and when the pressure of the cold bleed, or cold pressure, is constant, the flow delivered by the cold restriction is constant.
Thus, for a constant hot pressure and a constant cold pressure, the mixing rate of the FRV is constant, since it is the sum of two constant quantities, and its mixing ratio is constant since it is the ratio of two constant quantities.
When the hot pressure varies, the cold pressure remaining stable, the mixing ratio varies and the FRV becomes less accurate and less effective in moderating the return of the fuel mixture to the tank. However, in the prior art, a change in hot pressure is generally linked to that of the cold pressure. As a consequence, if the hot pressure varies, the prior art teaches inserting, in series between the hot bleed and the upstream inlet of the hot restriction of the FRV, a hot pressure regulator which can be used to regain the ideal operating conditions of the FRV under consideration with a constant pressure downstream of the hot pressure regulator. Since the pressure regulator is referenced to the cold pressure, the prior art FRV provided with a hot pressure regulator operates correctly when the difference between the hot pressure and the cold pressure can be kept constant. Such a fuel return valve provided with a pressure regulator at its upstream hot restriction inlet is termed a regulated FRV.
However, prior art pressure regulators, and thus FRVs provided with such a regulator, have limited efficiency when the hot pressure is stable and when the cold pressure, normally (in the case of architectures with a pressure recovery system between the hot bleed and the cold bleed, which are those being studied in this case. In architectures without a pressure recovery system, the cold pressure is ALWAYS higher than the hot pressure) less than the hot pressure, becomes greater than this hot pressure as well as variable.
In modern aeroplane engines, however, there are phases of a flight during which (inactive pressure recovery element) the difference between the hot pressure and the cold pressure, which is normally positive, becomes negative (presence of a pressure drop between the hot bleed and the cold bleed) and for which a FRV provided with a cold pressure that has become variable is still wanted.
Adding a second pressure regulator in series between the cold bleed and the upstream inlet of the cold restriction of a FRV cannot, however, be envisaged, since the pressure regulators cause leaks of fuel for the purposes of pressure regulation. In order to optimize the system, and in particular the dimensions of the fuel pump, it is standard practice to limit these leaks as much as possible. The presence of a cold line regulator and a hot line regulator thus appears in the prior art to limit the efficiency and simplicity of FRVs when considering and evaluating them as regards thermal management of the system which is germane to the invention.
For modern engines, then, integral management by oil/fuel or air/fuel heat exchangers is thus preferred. Thus, in the prior art, it is considered that FRVs cannot be used in modern engines, in particular because they are incapable of managing hot and cold pressure inversion phases when the hot pressure becomes lower than the cold pressure and when the cold pressure is stable, the hot pressure being highly variable (in a conventional flight, the cold pressure is stable while the hot pressure depends on the pressure recovery operation, which renders it unstable depending on whether or not it is used).
Despite high efficiency in certain phases of a flight and low bulk, the general use of regulated FRVs is thus considered in the prior art to be limited and they are not used for engines which are contemporary with that of the invention.